Method and apparatus for continuous exploration of bore holes



Nov. 28, 1939. J, J JAKOSKY 2,181,601

METHOD AND APPARATUS FOR CONTINUOUS EXPLORATION OF BORE HOLES Filed Dec.21, 1938 2 Sheets-Sheet 1 3mm John Jay Jakbsk y Nov. 28, 1939. J. J.JAKOSKY 1 L 0 METHOD AND APPARATUS FOR CONTINUOUS EXPLORATION 0F BOREHOLES Filed Des. 21, 1938 2 Sheets-Sheet 2 0 0 o o o o 0 '0 o o o 61 0 o///////////////1V/%:;mr /A%m J 01111 Jay Jakoslg y Patented Nov. 28,1939 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR CONTINU- OUSEXPLORATION OF BORE HOLES John Jay Jakosky, Los Angeles, Calif.

Application December 21, 1938, Serial No. 247,008

7 Claims.

This invention relates to an electrical method and apparatus fordetermining the character of the strata encountered at different depthsby a drill bit during the drilling of a bore hole.

A prior method, which employs a drill pipe having an insulated internalconducting tube, or an insulated conductor extending down inside of adrill pipe, and a bit which is insulated from the drill pipe, has beensuggested for this work. With such an arrangement the current flows fromthis insulated bit into the strata and through the earth to the otherterminal of the current circuit which is located at the surface of theground near the mouth of the hole, or it may be the drill pipe itself.Suitable measuring instruments are included in this circuit formeasuring variations in current and/or potential as the bit descends thehole. After repeated and extensive tests at commercial operations themethod has proven to be of limited use due to the difliculty of properlyinsulating the drill pipe from the bore hole itself, or of passing aninsulated wire down the inside of the drill pipe. Another practicaldifficulty in the latter method is to obtain proper insulation of thebit from the drill pipe. Materials having the desired insulatingqualities do not usually possess sufficient mechanical rigidity andruggedness to withstand the severe conditions encountered in drillingWork. In the method which is described in this invention, thesepractical difiiculties have been overcome, and in addition a simplifiedmethod of operation provided, which gives greater accuracy.

This application is a continuation-in-part of my application Ser. No.112,207, filed November 23, 1936.

An object of the present invention is to provide a method and apparatuswhich will give a visible, instantaneous, and preferably permanentrecord at the surface, as the drilling operations progress. Such recordsare of great value during drilling operations, especially in areas wherethe drilling must be stopped at a known horizon or strata, as forinstance when drilling into a high pressure oil sand, or when drillingthrough an oil sand, and where water may be encountered by a stratumunderlying the oil sand. By means of the present method and apparatussuitable indicating and signaling means may be located on the surface ofthe ground and the drilling operations supervised more intelligently.

Another object of the invention is to provide an advantageous method andapparatus for measuring the electrical conductivity of the strata whileutilizing either direct or alternating currents.

A further object of the invention is to provide means for measuring thevertical component of the natural earth potentials and other potentials5 while minimizing the effect of the horizontal component of the naturalearth currents on such measurements.

According to this invention a direct, alternating or other suitableelectric current is passed 10 through an electric circuit whichincludes. the drill stem and drill bit of a conventional drillingapparatus within a bore hole as one electrode, another electrodeconnected to the earth at a position laterally removed from the borehole, 15

changes with variations in the path of the cur- 20 rent flowing betweenthe drilling apparatus or bore hole electrode and the other or distantelectrode. In the preferred embodiments of the invention describedherein, this potential measurement is taken between two points spacedfrom one another along the path of current flow through the earth. Themeasurement is then repeated with the drill bit located at differentdepths within the bore hole in order to determine variations in thecharacter of the strata encountered at different depths by the drillbit.

The above-mentioned other electrode is sufficiently removed from thebore hole so that changes in the electrical characteristics of theformation adjacent the drill bit will produce variations in themeasurements. The positioning of this other electrode is an importantfeature of this invention. In general, it may be stated that when thisother electrode is connected to the earths surface it is preferablyspaced from the 40 the earth was to be energized between the drill bitand another electrode located closely adjacent the bore hole, or betweenthe drill bit and a direct connection to the surface casing, as shown bythe prior art, the current would flow largely between the near surface,and surface, drilling apparatus, and the adjacent surface electrode.Under such conditions a sufficient portion of the current would not fiowfrom the drill bit itself to give a readable or detectable change in themeasured quantity as the different strata are encountered by the drillbit.

The term measurements involving potentia as used throughout thespecification and claims, is meant to include the direct measurement ofa potential difference, the measurement of the relation between theenergizing current and a potential difference, as for example with anapparatus of one of the types shown in my copending applications SerialNo. 162,635, filed September 7,

1937, and Serial No. 172,009, filed October 30,

1937, or, when alternating current is used, the measurement, as oneexample, of the phase angle between the current in the energizingcircuit and the potential difference betwen the two points utilized inthe measurement. An apparatus for obtaining comparable phase-anglemeasurements is shown in my Patent No. 2,038,046.

In any electrical survey, when a measurement is taken while the earth isenergized by an electric current, the final interpretation of themeasurements depends not only upon the magnitude of the effect createdby the energizing current but also upon the magnitude or some othercharacteristic of the energizing current. Hence, it will be understoodthat throughout the specification and claims where mention is made ofmeasurements involving the potential difference, it will also beunderstood that the necessary information regarding the energizingcurrent will also be obtained. Such information regarding the currentmay be obtained by directly measuring the current or by controlling itsmagnitude as by keeping it substantially constant for example, or bydirectly measuring the ratio between the energizing current and thepotential difference.

As stated above, in practicing this invention the earth is energized byusing the drilling apparatus as one electrode, and it is not necessaryfor the drill bit to be insulated from the drill stem. Thus theenergizing circuit may be substantially the same as shown in myabove-mentioned copending application Serial No. 112,207 and also in mycopending application Serial No. 129,839, filed March 9, 1937, nowissued as Patents No. 2,153,802 and No. 2,150,169 (Re. 21,102)respectively. This invention may be practiced in bore holes in which thehole is filled or partly filled with a conductive liquid medium, such asan ordinary water clay mud drilling fluid in a manner comparable to thatdescribed in said copending application Serial No. 129,839, or with adrilling fluid of relatively low conductivity such as an oil claysuspension, or other oil or drilling fluids of low conductivity, in amanner comparable to that described in said copending application SerialNo. 112,207.

Further objects and advantages of the invention will become apparent asthe description proceeds.

I have diagrammatically illustrated apparatus according to my inventionin the accompanying drawings, in which:

Fig. 1 is a vertical cross-section of a portion of the earthillustrating the essential parts of a conventional rotary drillingapparatus in a borehole along with a circuit diagram according to myinvention;

Fig. 2 is another diagrammatic vertical section illustrating a modifiedcircuit arrangement;

Fig. 3 is a diagrammatic vertical cross-section of a portion of theearth showing a circuit arrangement which may be used to alternatelymeasure the natural earth potential and the combined natural earthpotential and the potential created by the flow of energizing current;in this case a cable drilling arrangement is shown, but the circuitarrangement is equally adapted to the rotary drilling equipment shown inthe other figures;

Fig. 4 illustrates a record which may be obtained with the apparatusshown in Fig. 3; and

Fig. 5 is a diagrammatic cross-section illustrating a modified circuitarrangement which may be used to obtain measurements comparable to themeasurements obtained with the apparatus shown in Fig. 3.

Fig. 1 illustrates the application of this invention to the conventionalrotary system of drilling. In this figure, the rotary table I isconnected to a proper source of motive power through shaft la. Therotary table, by means of the Kelly joint 2 transmits a rotary motion todrill stem 3 extending within a bore hole H. At the end of the drillstem is a drilling bit 4. The Kelly joint is supported at its upper endby a swivel-head l0. These parts constitute the usual type of rotarydrilling equipment and need not be described in detail herein. Aconnection terminal H is in electrical contact with the swivel-head andassociated drilling mechanism including the drill stem 3 and bit 4 andis associated with a suitable electrical energizing circuit, such ashereinafter described, for establishing an electrical circuit throughthe earth between the drilling apparatus and another electrode connectedto the earth at a position remote from the bore hole.

The present invention is also applicable to various types of rotarydrilling equipment and drilling bits, including various devices forobtaining rock samples, and to core bits. In addition, the invention maybe applied to cable drilling apparatus wherein a heavy bit assemblyimpacts and cuts into the earth by being alternately raised and loweredon a steel cable. This invention may also be applied to hydraulicdrilling methods employing a pipe, extending into the bore hole, fromwhich water under high pressure is being discharged. In short, themethod and apparatus of this invention may advantageously be employ-edwith any exploring and/or drilling apparatus which is constructed of, oremploys, electrically conductive materials.

Electrical connection to the rotary drill stem may be made by means ofterminal II, as above described and the other contact to the earth ismade by means of a suitable surface electrode 5, preferably of largecontact area, placed a considerable distance from the hole. Thisdistance is preferably one-third (or more) of the depth which thedrilling apparatus extends within the bore hole. In any case, theposition of this distant electrode is suffici-ently removed from thedrill hole so that an appreciable portion of the current will passthrough the drill bit 4 and through the earth formation adjacent thebit. The distant electrode 5 should be firmly imbedded in the earth insuch a manner as to minimize the contact resistance. I have found thatthe electrode should preferably be placed a distance of two to five feetbelow the surface of the ground. The casing of a distant well may beutilized as a distant electrode, or a suitable electrode of extendedsurface area may be lowered into contact with the water or otherconductive fluid in a distant well for this purpose.

A current source S and an indicating or recording ammeter 13 are shownconnected in series between the terminal I I and the electrode 5 throughthe agency of a suitably insulated conductor Id. The current source Smay comprise any controllable source of direct or alternating current ofeither high or low frequency, commutated direct current, or pulsatingcurrent, or any other suitable source of current. A potential-responsivemeasuring means M such as a conventional potentiometer is shownconnected between an earthed potential electrode 16 and a slidingterminal Ila in contact with the kelly 2 through the agency of asuitable insulated conductor ll. The kelly 2 is shown provided with acylindrical portion 2a providing a slip ring for the contact Ha. Thisprovides good electrical contact to drill stem and drill bit. Connectioncan of course be made directly to terminal on the swivel head ID. If aratio measuring instrument is used for the measuring means M, forexample one of the instruments described" in my abovementioned copendingapplications Serial Nos. 162,635 and 172,009, the current circuit isalso connected to the measuring means M. For example, one of the currentconductors l4 would be opened and the open ends connected through theinstrument M. The electrode l6 is preferably located within the path ofcurrent flow through the earth between the drilling apparatus and thedistant energizing electrode 5, and at a position spaced from thedrilling apparatus. It may be located in the same direction from thebore hole as the energizing electrode 5. The electrodes 5 and It may belocated fairly close to one another, on the order of feet for example,without interfering with the proper functioning of the apparatus, butthey are preferably located at a considerable distance from one another.For example, I may locate the electrode I6 in the opposite directionfrom the bore hole to the electrode 5. It should be understood, however,that the potential electrode [6 may be located considerably nearer tothe drill hole than it is possible to locate the current electrode 5 andstill obtain satisfactory operation of the apparatus.

According to one practice of the invention, the source of current S isset in operation and energizing current is caused to pass in, theenergizing circuit which includes drill stem 3 and drill bit 4 as oneelectrode, the energizing electrode 5 as the other electrode, and theportion of the earth.

included electrically therebetween. It will be seen that when anelectric current is passed through the earth between the drillingapparatus and the distant electrode 5, as by means of the abovedescribedenergizing circuit, the current will in some cases flow between theearth and the drilling apparatus throughout substantially the en'- tirelength thereof, particularly when the bore hole contains a conductivefluid medium such as the aqueous drilling fluid ordinarily employed inrotary drilling operations, as described in said copending applicationSerial No. 129,839. However, this fiow between the drill stem and theearth throughout the length of the drilling apparatus is not sopronounced when insulating drilling fluids are used such as brought outin said application Serial No. 112,207. A suitable drilling fluid whichmay fill or partly fill the bore hole is shown at F, and such fluid maybe of either the conductive or poorly conductive type. This liquidmedium F is maintained in the bore hole around the drill stem 3 which isexposed to electrical contact with such medium throughout substantiallyits entire length. It will be noted that the drill stem 3 is shownprovided with protectors P which may be of a type that is commerciallyavailable and are preferably of insulating material. These protectorstend to minimize the chance contacts of the drill stem with the wall ofthe bore hole and tend to prevent erratic variations in themeasurements. However, such protectors are not essential to the practiceof this invention and are shown only as a means of minimizing variationsin the measurements.

In any event, if the distant electrode is sufficiently removed from thebore hole, an appreciable portion of the current will flow throughoutsubstantially the entire length of the drill stem or other drillingapparatus extending within the bore hole, and through the strataadjacent the lower end of the drilling apparatus. It is this currentflow, which is appreciable if the distant electrode is removed asufiicient distance from the bore hole, which gives electricalvariations indicative of the nature of the strata adjacent the lower endof the drilling apparatus, as the drilling apparatus is moved todifferent depths. The resulting changes in current distribution producevariations in the magnitude and distribution of the potential fieldcreated by the current flow between the drilling apparatus and thedistant energizing electrode and produce variations in the potentialdifference between the terminal I la. and the electrode l5 which areindicative of variations in the electrical properties of the strataencountered by the drill bit at different depths. For practical purposesthe terminal Ha may be considered to be at substantially the samepotential as the drill bit 4.

Thus the variations in the measurements will be produced by changes inthe electrical characteristics of the formation adjacent the drill bitand when the variations in the measurements obtained on thepotential-responsive measuring means M are correlated with thecorresponding values of current in the energizing circuit as may beobtained from the ammeter 13, for example, valuable informationregarding the character of the strata encountered by the drill bit atdifferent depths may be obtained.

If direct current is employed, the readings obtained will be a functionof the resistance of the formations adjacent the drill hole, while ifalternating current is employed, the readings will be a function of theimpedance of such formations.

serve to indicate variations in the conductivity of the formations, theterm conductivity being considered herein as including either straightconductance (the reciprocal of resistance) in the case of directcurrent, or admittance (the reciprocal of impedance) in the case ofalternating current.

Measurements are preferably made continuously as the drilling proceedsin order to give a complete history of the drilling operations and toallow recognition of the various marker strata encountered by the drillbit as the drilling apparatus is extended to different depths in thebore hole and penetrates the formations successively at differentdepths. As brought out in the abovementioned copending applications, theelectrical information may be continuously recorded on a timeordepth-operated recording chart such as shown in said applications, sothat the information obtained may be used not only as an electritions.

cal record of the characteristics of the formation traversed by the borehole but also as a record of the operations of the drilling crew showingthe progress made in drilling and the time required to perform variousoperations such as removing the drilling apparatus from the hole.

The measurements should begin, however, at a time when the strataexhibit characteristic electrical changes which may be employed forcorrelation purposes. In some cases, important electrical markers(strata which may readily be recognized by characteristic changes inelectrical properties) manifest themselves many hundreds of feet abovethe oil sand, while in other areas important markers are encounteredadjacent to the oil sand. In metal mining operations, the measurementsshould begin prior to the time the drill penetrates the mineral-bearingforma- For location of water-bearing horizons the measurements alsoshould begin prior to the time the bit penetrates the water-bearingformations.

Referring to Fig. 2, a modified apparatus arrangement is shown in whichthe energizing circuit corresponds to the energizing circuit illustratedin Fig. 1 and the potential-responsive measuring means M is connectedbetween the potential electrode I6 and a second potential electrode i6a,both connected to the surface of the earth and spaced from one anotheralong the path of current flow through the earth. Thus the conductor Hinstead of being connected to the terminal I la which is connected tothe drill stem 3 is connected to the additional potential electrode l6a.With the arrangement illustrated, the electrodes 5, I6, and Ilia areshown arranged substantially along a single straight line which passesthrough the bore hole H and the electrodes i5 and Mia are positionedbetween the bore hole and the distant energizing electrode 5. It is tobe understood that this particular arrangement of the electrodes is notnecessary to the satisfactory operation of the apparatus, but that theelectrodes l6 and Mia may be connected between any two points whosepotential difference changes with variations in the path of the currentflowing between the drilling apparatus and the distant energizingelectrode 5. It is preferable, however, to have the potential electrodeslocated at a considerable distance from the energizing electrode 5. Forexample, they may be located in the direction from the bore holeopposite to the direction of the electrode 5.

Measurements may be taken with this apparatus in exactly the same manneras with the apparatus shown in Fig. 1. Thus the earth is energizedbetween the drilling apparatu and the electrode 5, and measurements aretaken involving the potential difference between electrodes l6 and Iliawith the drill bit 4 located at different depths within the bore hole.

The measurements obtained with the apparatus shown in Figs. 1 and 2 willin general be affected by the varying natural earth potentials, and inFig. 3 I have shown a way for minimizing the effects of such potentialson the final measurements. In this figure a churn or cable drillingapparatus is illustrated and is shown as comprising a heavy bit assemblyincluding a drill bit M and a connected drill stem 45 suspended withinthe bore hole H by a cable 45a. With such an arrangement the bit isalternately raised and lowered by the cable 4511 by means not shown, sothat as the bit is lowered it impacts the earth and produces the hole H.The energizing circuit is shown as comprising a switching means 54 andpower source S connected in series between distant energizing electrode23 and the drilling assembly including the drill bit 44 throughinsulated conductors l0 and H. The conductor H is connected by a clampHa to cable 45a which provides electrical connection to the drill bit44. The source S is preferably a constant current generator providedwith suitable manual or automatic means for maintaining a constantcurrent reading on the ammeter A during the times of current flow,although any other suitable controlled source may be used. The switchingmeans 54 may comprise stationary quadrants 55 and 56 and a rotor bar 51connected to a suitable time-operated shaft so that as the time-operatedrotor 51 turns, connection will first be established between thequadrants 55 and 56 by the bar 51 and then broken so that periods ofcurrent flow are alternated with periods of no current flow.

The potential circuit is somewhat modified, and is shown as comprising apotentiometer 52 and measuring means M connected in series withpotential electrode 22 and the drilling assembly by conductors 12 and13, respectively. The conductor 13 is shown connected to cable 45athrough clamp 13a. The natural earth potential between the drill stemand the electrode 22 may now be measured with the portion of theelectrical circuit described above. The electrode 22 may be located withrespect to the bore hole and the distant energizing electrode 23 in anyof the manners described in connection with the electrodes 5 and I6.

It will be understood that the circuit arrangement above described isequally adapted to use with rotary drilling equipment as illustrated inFigs. 1 and 2 for example. Thus the conductors H and 13 instead of beingconnected to the cable 4511 would be electrically connected to the drillstem 3 illustrated in Fig. 1, for example.

In order to establish a suitable zero level on the record of therecording meter M which is indicated at 5! in Fig. 4, the potentiometer52 may be adjusted to bring the indication of the earth potential to anyconvenient point on the recording medium 5|. It will be appreciated thatas the bit 44 bores into the earth, the potential will vary or drift andit may be necessary from time to time to readjust the potentiometer 52to keep the potential readings on the record; also when bad groundcurrents prevail it may be necessary to reverse the polarity of thebattery 5| in some instances to maintain the desired control of therecording.

When connection is established between the quadrants 55 and 56 themeasuring means M will give indications which are indicative of thepotential between the drilling apparatus and the electrode 22 asinfluenced by the current flow from 23 to the drilling apparatus; theseindications will also be influenced by the natural earth potential. Whenconnection is broken between said quadrants the meter M will giveindications of natural earth potentials as above described.

Referring particularly to Fig. 4, I have shown an example of a recordwhich may be obtained with the apparatus shown in Fig. 3. The solid line62 on the recording tape 6| represents the path of the stylus of themeasuring means M; the substantially horizontal portions of said record,indicated at 62a, represent the interval at which the rotor 51 eithermakes or breaks with the quadrants 55 and 56. For example, the shortportions of the recording on the left side, indicated at 62b, mayrepresent the value of the natural earth potential during an interval inwhich the rotor 51 of the switching means 54 does not establish contactwith the quadrants 55 and 56 when the rotor 51 contacts said quadrantsthe stylus will move across the record as at 62a and trace a record, forexample of potential, at 620 for the length of time said connection ismaintained. At the end of said time the contact will be broken and thestylus will move to the left along 62b and trace the curve 62a. Thus itmay be seen that by dotting in the lines between the adjacent portionsof the curves 62b and 620, a continuous record of the earth potentialand potential indicative of earth conductivity or other property and thenature and characteristics of the strata may be plotted simultaneouslyon the single recording medium.

It will be appreciated that suitable compensating networks may beinserted in the circuit of the measuring means M and in operativeassociation with the switching means 54 for adjusting the relativepositions and amplitudes of the earth potential curve 625 and potentialcurve 620 as will readily be apparent to those skilled in the electricalart.

It will be apparent that the conductor 13 may be connected to a suitablepotential electrode comparable to the electrode |6a shown in Fig. 2instead of to the drilling apparatus, If such is the case measurementswill be made between two potential electrodes as shown in Fig. 2.

It will be appreciated that the power source S shown in Fig. 3 may bealternating current source in which case the recording means M may bereplaced by a suitable direct current instrument for obtaining naturalearth potential readings and an alternating current instrument forobtaining alternating current potential readings. Circuit arrangementssuitable for obtaining such readings are shown and described in PatentNo. 2,132,807 to Rust, Jr., et al.

An alternative arrangement for minimizing the effects of natural earthpotential on the final measurements is illustrated in Fig. 5. Distantpotential electrodes 2| and 22 are located at a considerable distancefrom the drill hole H and are shown connected through suitable insulatedconductors 4| and 42 to a common conductor 43 which is connected to thevariable tap of a potentiometer 52. A drill bit 14 is shown in contactwith the bottom of the hole H and is in electrical connection with adrill stem 15 which is connected through a suitable conductor 46 to oneside of a potential responsive measuring device M such as a conventionalindicating or recording potentiometer, the other side of said measuringdevice being connected to the other side of the potentiometer 52 througha conductor 41. The bore hole H may be filled or partly filled with afluid F which may be conductive or it may be partly insulating asdescribed in the above-mentioned application Serial No. 112,207. Thepotential between the conductors 43 and 41 may be made variable byconnecting a battery 5| across the potentiometer and varying thepotentiometer 52 according to well-known procedures.

The utilization of two or more potential electrodes connected togetherat a common point, such as the electrodes 2| and 22, connected togetherby the conductors 4| and 42 to the common conductor 43, is one importantfeature of this invention. Such an arrangement tends to eliminatepotential variations due to changing earth currents, which currents areordinarily a varying source of error and are present at all times,making it practically impossible to duplicate survey results. Thisfeature has been particularly pointed out in my pending applicationSerial No. 103,038, filed September 28, 1936, and. in my above-mentionedcopending application Serial No. 112,207, and it will suffice to saythat the reference electrodes such as 2| and 22 should be spaced withrespect to hole H and connected together in such manner that thehorizontal component of the natural earth currents in the neighborhoodof the bore hole will have a negligible efiect on the referencepotential no matter what the direction or magnitude of the earthcurrents. In areas where extreme variations in earth currents areencountered, more than two potential electrodes may be advantageouslyemployed. It has been found that normal earth currents fiowsubstantially horizontally, and have a very minor vertical component. Inthe example illustrated in Fig. 5, the electrodes 2| and 22 are shownsubstantially equidistant from the drill hole H and at a considerabledistance therefrom, and that 2|, 22, and H are substantially in a linewith one another. The electrodes 2| and. 22 may be disposed preferably!symmetrically with respect to the bore hole, on a straight line at rightangles to the straight line joining the bore hole and the distantenergizing electrode.

The natural earth potential between the reference potential supplied bythe points 2| and 22 and the drill stem 15 may now be measured with theportion of the electrical circuit described above. Since the electrodes2| and 22 are located at a considerable distance from the drill hole andare connected together through conductors 4| and 42, the juncture ofsaid conductors and the conductor 43 is at a more or less averagepotential of the points 2| and 22 and is considered as a referencepotential, and measurements may be taken with the measuring device M,which may comprise a recording potentiometer, which are indicative ofthe vertical component of the earth potential at the position of thedrill bit 14.

I have also shown a distant energizing electrode 23 connected through aninsulated conductor 53 including an ammeter A to a variable potentialpower source S, the other side of said source S being connected to oneside of a switching means 54 and the other side of said switching meansbeing connected through a conductor 58 to the conductor 46 which isconnected to the drill stem 15.

The distant energizing electrode 23 is placed at a considerable distancefrom the bore hole H, for example a distance equal to or greater thanone-third the depth to which the bit 14 is carried, so that suflicientenergy will flow in the earth to create a measurable potential betweenelectrodes 2| and 22, and the bit 14 or drill stem 15. The electrodes 2|and 22 may be disposed, preferably symmetrically with respect to thebore hole, on any one of a number of straight lines bearing any angularrelation to the straight line passing through the bore hole H and theelectrode 23. For example, the electrodes 2| and 22 may be disposed on astraight line perpendicular to the straight line passing through thebore hole and the electrode 23. At least one, and preferably both, ofthe electrodes 2| and 22 are located within the path of current throughthe earth between the drilling apparatus and the electrode 23.

It may be readily seen that current flow through the earth between thedrilling apparatus and the electrode 23 will influence the potential ofadjacent points on the surface of the earth and that the potential ofany of said last-named points will be a function of the currentdistribution through the earth, which is indicative of the nature of thestrata, and a function of the current in said circuits. By maintaining aconstant current through the earth circuit interpretation will begreatly simplified, although results may be interpreted if variations incurrent are recorded along with potential measurements. If variablecurrent energization is employed, the meter A should preferably be of arecording type, to allow proper interpretation of the potential recordmade by recording meter M.

It is not entirely necessary to use the switching means 54 so as toobtain alternate measurements involving the natural earth potentialalone when no current is flowing and measurements involving the combinedcreated and natural earth potentials during the flow of energizingcurrent as described in connection with Figs. 3 and 4,

since the use of the reference potential electrodes 2| and 22 minimizesthe effects of the natural .earth potentials. However, alternatemeasurements may be taken in the same manner as described in connectionwith Fig. 3 to obtain a record which may be entirely comparable to therecord illustrated in Fig. 4.

Methods and apparatus in which measurements are taken indicative of thepotential during successive energizing current pulses and during theintervals between such current pulses as described above are more fullydescribed and claimed in my copending application Serial No. 230,963,filed September 21, 1938.

It is to be understood that the circuit arrangements illustrated inFigs. 1, 2, and 5 are equally adapted for use with cable drillingequipment as illustrated in Fig. 3, in which case the circuitconnections which are made to the drill stem either directly or throughthe swivel-head are made to cable 45c as illustrated in Fig. 3.

In general, the magnitude of the variations in the measurements producedby changes of given magnitude in the electrical characteristics of thestrata will change as the drilling apparatus is extended to greaterdepths. For example, a given change in characteristics of the strata maygive a variation of ten per cent in the measurements when the drill bitis at one depth and only a five per cent change when the drill bit is ata greater depth. Hence, this phenomenon must be taken into considerationif it is desired to directly compare the measurements at relativelyshallow depths with the measurements at relatively great depths. Inpractice the measurements at different depths may be multiplied byempirical factors based upon experience in the area, in order to permitsuch comparison.

I claim:

1. In the drilling of a bore hole with a drilling apparatus comprising adrill stem extending within the bore hole and a drill bit electricallyconnected to said drill stem at the lower end of said drill stem, themethod of determining the character of the strata encountered atdifferent depths by the drill bit during the drilling of said bore hole,which comprises: passing a controlled electric current through anelectric circuit including the drill stem and the drill bit as oneelectrode, another electrode connected to the earth at a positionremoved from the bore hole, and the portion of the earth includedelectrically between said electrodes; taking a measurement involving thepotential difference between two points whose potential differencechanges with variations in the path of the current flowing between saidelectrodes; and repeating such measurement with said drill bit locatedat different depths within the bore hole, while maintaining a liquidmedium in the bore hole around said drill stem, said drill stem beingexposed to electrical contact with said liquid medium throughoutsubstantially its entire length and said other electrode beingsufficiently removed from the bore hole so that changes in theelectrical characteristics of the formation adjacent the drill bit willproduce variations in said measurements.

2. In the drilling of a bore hole with a drilling apparatus comprising adrill stem extending within the bore hole and a drill bit electricallyconnected to said drill stem at the lower end of said drill stem, themethod of determining the character of the strata encountered atdifferent depths by the drill bit during the drilling of said bore hole,which comprises: passing a controlled electric current through anelectric circuit including the drill stem and the drill bit as oneelectrode, another electrode connected to the earth at a positionremoved from the bore hole, and the portion of the earth includedelectrically between said electrodes; measuring the potential differencebetween two points whose potential difference changes with variations inthe path of the current flowing between said electrodes; and repeatingsuch measurement with said drill bit located at different depths Withinthe bore hole, while maintaining a liquid medium in the bore hole aroundsaid drill stem, said drill stem being exposed to electrical contactwith said liquid medium throughout substantially its entire length andsaid other electrode being suificiently removed from the bore hole sothat changes in the electrical characteristics of the formation adjacentthe drill bit will produce variations in said measurements.

3. In the drilling of a bore hole with a drilling apparatus comprising adrill stem extending within the bore hole and a drill bit electricallyconnected to said drill stem at the lower end of said drill stem, themethod of determining the character of the strata encountered atdifferent depths by the drill bit during the drilling of said bore hole,which comprises: passing a controlled electric current through anelectric circuit including the drill stem and the drill bit as oneelectrode, another electrode connected to the earth at a positionremoved from the bore hole and the portion of the earth includedelectrically between said electrodes; measuring the potential differencebetween a pair of potential electrodes connected to the earth at twopoints whose potential difference changes with variations in the path ofthe current flowing between said electrodes; and repeating suchmeasurement with said drill bit located at different depths within thebore hole, while maintaining a liquid medium in the bore hole aroundsaid drill stem, said drill stem being exposed to electrical contactwith said liquid medium throughout substantially its entire length andsaid other electrode being sufficiently removed from the bore hole sothat changes in the electrical characteristics of the formation adjacentthe drill bit will produce measurable variations in said measurements.

4. In the drilling of a bore hole with a drilling apparatus comprising adrill stem extending within the bore hole and a drill bit at the lowerend of said drill stem, .the method of exploring the strata traversed bythe bore hole, which comprises: passing an electric current through theearth between the drill stem and drill bit as one electrode and anotherelectrode connected to the earth at a position remote from the borehole, and measuring an electrical potential associated with such currentflow; interrupting said current flow and taking a measurement of naturalearth potential while said current flow is so interrupted; and repeatingthe steps of alternately measuring one potential during current flow andthe other potential when the current flow is interrupted as said drillstem is extended to different depths, said other electrode beingsufficiently distant from said bore hole so that variations in themeasured electrical potential associated with the flow of such currentwill be produced by changes in an electrical characteristic of thedifferent portions of the earth ineluded electrically between saidelectrodes as said drill bit is moved to different depths in said borehole.

5. An apparatus for use in determining the character of the strataencountered at different depths by the drill bit during the drilling ofa bore hole containing a liquid medium, which comprises: an electrodecomprising a drill stem and the drill bit electrically connected to thedrill stem and in electrical connection with the earth within said borehole, said drill stem being exposed to electrical contact with saidliquid medium throughout substantially its entire length; an electrodeconnected to the surface of the earth at a position remote from saidbore hole; a controllable source of electric current connected to saiddrill stem and to said last-named electrode for passing an electriccurrent through the earth between said electrodes; andpotential-responsive measuring means connected between two points whosepotential diiference changes with variations in the path of the currentflowing between said electrodes for indicating changes in an electricalcharacteristic of the earth included between said electrodes when saiddrill bit is located at different depths, said other electrode beingsufiiciently distant from said bore hole so that variations in saidmeasurements will be produced by changes in an electrical characteristicof the different portions of the earth included electrically betweensaid electrodes as said drill bit is moved to different depths in saidbore hole.

6. An apparatus for use in determining the character of the strataencountered at different depths by the drill bit during the drilling ofa bore hole containing a liquid medium, which comprises: an electrodecomprising a drill stem and the drill bit electrically connected to thedrill stem and in electrical connection with the earth within said borehole, said drill stem being exposed to electrical contact with saidliquid medium throughout substantially its entire length; an electrodeconnected to the surface of the earth at a position remote from saidbore hole; a controllable source of electric current connected to saiddrill stem and to said lastnamed electrode for passing an electriccurrent through the earth between said electrodes; andpotential-responsive measuring means connected between a pair ofelectrodes electrically connected to the earth at two points whosepotential difference changes with variations in the path of the currentflowing between said electrodes for indicating changes in an electricalcharacteristic of the earth included between said electrodes when saiddrill bit electrode is located at different depths, said other electrodebeing sufficiently distant from said bore hole so that variations insaid measurements will be produced by changes in an electricalcharacteristic of the different portions of the earth includedelectrically between said electrodes as said drill bit is moved todifferent depths in said bore hole.

'7. An apparatus for use in determining the character of the strataencountered at different depths by the drill bit during the drilling ofa bore hole, which comprises: an electrode comprising a drill stem andthe drill bit positioned within a bore hole and in electrical connectionwith the earth therein; an energizing electrode connected to the surfaceof the earth at a position laterally removed from said bore hole; powersupply means connected to said energizing electrode and to said drillstem through switching means operable to alternately connect anddisconnect said power supply means to said drill stem; potentialelectrode means in electrical contact with the earth; and measuringmeans connected between said last-named electrode means and said drillstem for indicating variations in the natural earth potentials betweensaid drill stem and said other electrodes as said drill bit is moved todifferent depths in the bore hole, said measuring means being operableto indicate variations in natural earth potentials when said switchingmeans is in disconnected position and operable to indicate variations inan electrical characteristic of the earth included between saidenergizing electrodes when said switching means is in connectedposition, as said drill bit is moved to different depths within the borehole.

JOHN JAY JAKOSKY.

